Posts Tagged animal diversity

http://www.youtube.com/watch?v=HBxn56l9WcUI’ve always liked frogs. I remember, when I was probably around 4 years old, being fascinated by the tadpoles that Dad brought home in a big jar from a farm pond. Mum explained about how they’d gradually metamorphose (thought I doubt she used that word!) & we watched their legs slowly grow & their tails disappear as they swam around in an old tub, until the point where they became frogs. Frogs are amphibians, along with newts & mud-puppies & axolotls and the legless caecilians (which look like a cross between an eel and an earthworm). As a group, frogs are much younger – in geological terms – than the others: most fossil frogs date back only about 50 million years, although the earliest-known frog-like creature, Triadobatrachus, lived about 250 mya in the early Triassic. Like almost all terrestrial amphibians, adult frogs use not only lungs for gas exchange, but also their skin and the membranous lining of their mouths. (Lungless salamanders are an exception – as the name suggests, they must rely on their skin alone, which is very convenient for those researching amphibian gas exchange.) This reliance on transcutaneous respiration has meant that amphibians are very susceptible to harm due to to chytrid fungus infection, which severely damages the skin and markedly reduces the animals’ ability to exchange O2 & CO2 with the atmosphere. In addition, using your skin as a gas exchange surface means that you have to keep it moist. This means that we’d expect to find frogs only in environments that are humid and damp year-round, & in general that’s the case. But there are always exceptions. and the desert rain frog is one of them. Breviceps macrops lives in one of the most inhospitable environments there is, a dry coastal strip of land in Namibia & South Africa. Hardly a place for a frog! It spends most of its time in burrows dug deep enough to reach into moist sand, but comes out at night when the air is cooler & more humid. While there’s very little actual rain, moisture-bearing sea fogs roll in from the ocean on at least 100 nights each year, bringing some water to the habitat as the fogs condense onto dunes & vegetation – enough to allow these little amphibians to survive. (There’s no actual tadpole stage in their life cycle; little froglets develop directly from eggs in the burrows.) And like other amphibians, they vocalise to advertise their presence. I hesitate to say the sound is a croak. In fact, it drove my dog to distraction when I played the following clip. I give you – ‘the sonorous war cry of a very angry frog‘.

In fact, the creature shown in this gorgeous image by Daniel Llavaneras is neither mantis nor true (Dipteran) fly, although its common name is ’mantisfly’. Instead, it belongs to the insect family Mantispidae (a group that includes lacewings and antlions). Like real praying mantids, matisflies walk on 4 legs, with the front pair folded as shown, and the head is somewhat mantis-like. The adults hunt as mantids do, shooting out those raptorial front legs to catch small insects, while the larval diets vary: some are also active predators, while others consume wasp & beetle larve, or spider eggs (later pupating in the spider’s egg sac). In adult form & behaviour, the mantisflies are an excellent example of convergent evolution.

Seeing this image of a fish with 2 mouths reminded me that I needed to finish writing about Frankenlouie, a janus-headed (diprosopic) cat. It’s funny how the mind works, because the fish definitely isn’t a janus-fish: that would require the mouths to be side-by-side rather than one above the other. (While this is a rather unsightly mutation, the fish seems to have survived in the wild until a fisherman hauled it out.)

Photo: Garry Warrick)

So – on to Frank’n'Louie.

Frank’n'Louie was (were?) described as a ‘janus cat’ because he had two faces that looked in different directions, like the Roman god Janus (as opposed to that fish, which has two mouths one atop the other). Many people would have found him rather hard to look at, as he had 3 eyes, the middle one of which was blind; two noses; two mouths; and but a single brain. The fact of that single brain means, I suppose, that this really was one, strange-looking, individual cat, rather than the two distinct individuals seen in dicephalic parapagous conjoined twins such as the Hensel sisters. Despite being expected to die soon after birth, Frank’n'Louie attained the ripe old age of 15 years before succumbing to cancer in 2014.

Frankenlouie’s features are the result of craniofacial duplication, or diprosopus: an individual with a single body and normal limbs, but a greater or lesser degree of duplication of the face. (He was lucky to survive so long as many janus individuals also have neural tube defects, including – at their most severe - anencephaly, or the absence of a brain, and die very young). When I first saw a picture of this cat I wondered if his features had something to do with conjoined twinning, and apparently that’s often put down as the underlying cause if the organism has two complete faces.

However, another possible cause is a mutation in the gene responsible for the Sonic Hedgehog protein (SHH), which among other roles is involved in the control of craniofacial development. Too much of that protein (overexpression of the mutant form of the gene) results in craniofacial duplication; too little can cause cyclopia, where there is just a single eye. (Infants with cyclopia die soon after birth as the condition is associated with severe brain abnormalities, so the Cyclops of the Ulysses stories would not have been modelled on an actual adult with the condition.)

In fact, SHH plays a crucial role in embryonic development, as this description on the National Institutes of Health gene database makes clear:

It has been implicated as the key inductive signal in patterning of the ventral neural tube, the anterior-posterior limb axis, and the ventral somites.

This means that mutations in the gene coding for SHH can have far-reaching impacts on the development of the brain and nerve cord, limbs, and body segments, while a mutation in one of the enhancer regions (an enhancer is a region on a chromosome that affects transcription of a particular DNA sequence) results in duplication of the thumb.

But there’s more: Sonic Hedgehog is one of a group of ‘evolutionarily conserved’ genes (others in this gene family include ‘Desert Hedgehog’ (!) and Indian Hedgehog) found in vertebrates, so SHH is involved in the patterning of embryo development in all vertebrates, not just in mammals like Frankenlouie. These ‘conserved’ regions of DNA tend to play crucial roles in development and functioning of an organism, and so are relatively unchanged over time: any significant alterations in their sequence, and so in their products, would probably be subject to strong negative selection. And Sonic Hedgehog’s gene family is in turn related to the hedgehog gene that is involved in proper formation of body segments in Drosophila. So the chromosomal region that’s most likely to be implicated in Frankenlouie’s particular birth defect is one with a very long evolutionary history indeed, one that extends back beyond the split between invertebrate and vertebrate lineages.

One of the things we talk about in biology class is the importance of decomposers. Most students think in terms of bacteria when this topic’s raised, & maybe things like fungi. But there is more to the breakdown of a body than those microorganisms.

Think worms, for example. In his final bookA, Charles Darwin highlighted the significant role played by earthworms in breaking down ‘vegetable matter’ (eg leaves) to produce what he called ‘vegetable mould’.

And of course there are ants. While we may think of them as those irritating little critters that overrun the kitchen if they find a food source, & produce anthillsB of sand in the cracks in paving, they also act as what could be called macro-decomposers. As this video demonstrates:

Lizards, like us, are chordates. One of the defining characteristics that all chordates share at some point in their development is the presence of a notochord: a stiff rod of tissue that runs along the dorsal side of the animal, just beneath the hollow dorsal nerve cord. (Yes, hollow. This is the result of its origins in the neural tube that forms early in chordates’ embryonic development.) In most vertebrates the notochord’s replaced by the spinal column. Another chordate feature is the presence of pharyngeal pouches (homologous to gills in fish, and to structures in the jaw and inner ear in mammals), and there’s also the tail. A tail that extends beyond the anus. And it’s that last fact that sets lizards & scorpions apart, when it comes to losing their tails.

This ability to shed the tail is known as autotomy, and it seems to have evolved in response to predator pressure: the tail may even continue to wriggle for a while, which would help to distract a carnivore long enough for the lizard to escape and to live another day.

And that longer-term survival post-autotomy has much to do with the fact that a chordate’s tail is ‘post-anal’. For when a lizard (eg a gecko, or a skink) loses its tail, the animal’s gut remains intact; it can continue to take food in at one end & pass faeces out the other.

Scorpions are arachnids, related to spiders and mites. As a paper published earlier this year in PLoS ONE notes (Mattoni et al, 2015), scientists have known about autotomy in arachnids, but up until now they’d only observed the voluntary loss of legs. However, Mattoni & his co-workers augmented data from the field, and from museum specimens, with some (very careful!) experiments on live animals to demonstrate that at least some species of scorpions are able to detatch their tails.

As for lizards, a tail (more correctly, a ‘metasoma’) continues to wriggle for a while after it’s detatched, and may also act as a distractor to allow the animal to escape a predator. There is, however, a drawback – with its tail the scorpion also loses its anus and the penultimate portion of its digestive tract. And neither metasoma nor gut regenerates.

On the face of it, you have to wonder why caudal autotomy (the ability to voluntarily shed the tail) would ever have been selected for in scorpions. They’re unable to sting ever again, which would leave them with a much-reduced ability to defend themselves or to kill large prey items. And once the open end of the intestine is closed by scar tissue – which takes about 5 days – they can no longer pass faeces from the gut, which must put a dampener on their ability to take food in at the other end – a case of enforced constipation? (The authors note that in at least some cases, the pressure of accumulating poo may trigger another autotomic event, when the animal loses the segment at the ‘new’ end of the tail.)

However, for the scorpions, all was not lost. The researchers’ lab experiments showed that the tail-less arachnids still managed to survive for up to 8 months post-amputation, occasionally eating small prey items. Which would be irrelevant if they were unable to pass their genes on – but the animals were also able to reproduce. In mating experiments, tail-less males were nonetheless able to court and mate with females on multiple occasions. This means that tail-shedding may still provide a selective advantage, in that it allows animals to escape predation and go on to reproduce.

A lot of my friends seem to like owls, if their tendency to post photos of adorable fluffy feathered faces on Facebook is anything to go by. I rather like them too; we live close to a gully & it’s lovely hearing the moreporks calling at night. Once or twice one has sat in a tree just outside our window – very special!

Of course, behind the beauty lies a fierce, predatory nature, and that is well captured (in a most humorous way) in this video from the wonderful ‘True Facts’ series:

I do not remember reading any fairy tales involving the ripping off of small persons’ faces by an owl. I’m sure he just made that bit up!

When I first saw an image of this stunning bird (on FB, as one might expect) I thought I was looking at the male of a strongly dimorphic species. However, it turns out that both sexes share this spectacular colour pattern, although the colours may be somewhat muted in females. They’re easier to distinguish in the breeding season, because the red & yellow lumps, or nodules, that dot the head & neck swell in males & become even more brightly coloured.

Most conservationists consider it near-threatened, with deforestation making the birds easier to kill by local subsistence hunters, a major factor in the species’ decline.

The North American wild turkey got pushed close to the brink of extinction in New York state & has since bounced back due to careful management of the population and it’s habitat, so there’s hope for its gorgeous cousin if suitable conservation mechanisms can be identified & put in place.

In her book Paleofantasy, Marlene Zuk discusses cane toads (Bufo marinus) as an example of just how rapidly evolutionary processes can work. These amphibian pests were introduced into Australia in 1935 to control borer beetles in sugar cane. Unfortunately the toads never got the memo about this expectation, and have spread rapidly across the continent, damaging a range of native ecosystems as they go. (They’re aided by the fact that they’re toxic, killing many of the predatory animals that might otherwise eat them.)

When the toads were first introduced, they spread at a rate of about six miles (ten kilometers) per year. Today cane toads advance more than 31 miles (50 kilometers) annually.

In other words, they’re getting faster, with animals at the ‘invasion front’ moving up to 1.8km in a night. (The researchers were able to measure the toads’ speed by fitting them with miniature radiotransmitters, strapped to their waists.) Phillips & his colleagues (2006) point out that speed of movement in toads is correlated with leg length, and asked the question: is there a difference in average leg length between toads at the front of the amphibian wave spreading across Australia, and those at the back of the bunch? The answer:

And the evolutionary changes don’t stop there. In a paper just out, Brown, Phillips & Shine (2014) describe how the animals’ tendency to travel in a straight line has changed too:

Radio-tracking of field-collected toads at a single site showed that path straightness steadily decreased over the first 10 years post-invasion.

The research team found that this behavioural change had a genetic underpinning. The progeny of toads from the invasion front moved in straighter paths than the offspring of toads from older, well-established populations to the east. In addition, “offspring exhibited similar path straightness to their parents.” Brown & his colleagues concluded that

The dramatic acceleration of the cane toad invasion through tropical Australia has been driven, in part, by the evolution of a behavioural tendency towards dispersing in a straight line.

Over the last 20 years quite a bit of evidence has accumulated indicating that at least some dinosaurs were feathered, much of it in the form of beautiful fossils from China. Up until now all the feathery dinos have been members of the carnivorous theropods, but this new paper by Godefroit et al (2014) extends that fluffiness in its description of a herbivorous dinosaur, Kulindadromeus zabakialicus. (The full paper is behind a paywall but the BBC offers a good general summary.)

It’s now generally accepted that birds evolved from a theropod lineage (Michael Benton discusses the evolutionary changes that this entailed, here), although there is still debate around the origins of things like wings, feathers, and when birds/dinos first took to the air. Most people are probably familiar with at least the name of Archeopteryx, but since 1994 those Chinese fossils have shown us that many more theropods were feathered, and that feathers evolved well before the first bird-like creatures took to the air. Godefroit & his colleagues comment that

fully birdlike feathers orginated within Theropoda at least 50 million years before Archaeopteryx.

But Kulindadromeus wasn’t a theropod – it was a ‘neornithischian’ – an early member of the ‘bird-hipped’ dinosaurs, a group that includes Stegosaurus and Triceratops. (This nomenclature can get a bit confusing, especially when you consider that birds evolved from ‘saurischian‘, or ‘lizard-hipped’ dinos.) And while it didn’t have the sort of feathers that we’re familiar with today, it did have a range of other structures in addition to the usual scales:

monofilaments around the head and the thorax, and more complex featherlike structures around the humerus [upper forelimb], the femur [thigh], and the tibia [lower leg].

It’s early days yet. But if other ornithischians are found with feathers, then then this would raise the possibility that the common ancestor of both dino groups also had some sort of feathery structures on its body, and would support the authors’ suggestion that

feathers may thus have been present in the earliest dinosaurs.

In other words, feathers may well be much, much older than we’ve thought.

About SciBlogs

Sciblogs is the biggest blog network of scientists in New Zealand, an online forum for discussion of everything from clinical health to climate change. Our Scibloggers are either practising scientists or have been writing on science-related issues for some time. They welcome your feedback!